The goal of the proposed studies is to define the electrophysiologic and subcellular mechanisms underlying nonreentrant initation of ventricular tachycardia (VT) in the failing heart and its modulation by adrenergic stimulation. In the preceding grant interval, we have performed 3- dimensional mapping studies in arrhythmogenic experimental models of cardiomyopathy and in the failing human heart and demonstrated that VT initiates by a nonreentrant mechanism that is enhanced by catecholamines. The applicant has isolated myocytes from failing hearts and found alterations in Na/Ca exchange activity and intracellular calcium handling that could underlie the development of an arrhythmogenic transient inward current (Iti). Studies will be performed both in an arrhythmogenic rabbit model of nonischemic cardiomyopathy and in the failing human heart. The contribution of Alpha1-, Beta1- and Beta2-adrenergic receptor stimulation to arrhythmogenesis in the failing heart will be determined by in vivo 3-dimensional mapping and in vitro electrophysiologic studies. Measurement of Alpha1-, Beta1-, and Beta2- adrenergic receptor density with microscopic resolution using autoradiographic techniques will determine whether the density of adrenergic subtype receptors parallel the arrhythmogenic effects of adrenergic subtype stimulation. To delineate how alterations in sarcoplasmic reticulum (SR) calcium flux, Na/Ca exchange activity and a calcium-activated chloride current lead to activation of a Iti in the failing heart, and to determine how the activation of Iti is enhanced by adrenergic stimulation, whole cell voltage clamping and measurement of intracellular calcium and SR calcium content will be performed in myocytes isolated from myopathic hearts. Lastly, to determine whether nonreentrant activation is due to triggered activity arising from delayed afterdepolarizations (as opposed to early afterdepolarizations or abnormal automaticity), studies will be performed in a novel isolated heart preparation in which transmural mapping in vitro will be combined with recording of monophasic and transmembrane action potentials. The results of these studies will provide new insights into the nature of nonreentrant activation in the failing heart and of the subcellular alterations that underlie adrenergic enhancement of arrhythmogenesis. The results will also provide the foundation for novel therapeutic approaches directed at nonreentrant activation that would be useful in the prevention of sudden death in patients with cardiomyopathy.